High salt-containing water is available worldwide in sea water, underground water and other sources. These sources of water, although available in great abundance are practically useless because high-salt content water is non-potable, therefore unfit for human and animal consumption or the chemical species constituting the salt is toxic to vegetation, therefore rendering the water unsuitable for crops, fruit trees or any other vegetal growth or the high salt content renders the water corrosive, therefore rendering the water unsuitable for industrial use lest corrosion of equipment, pipes and other metal parts would require frequent and costly replacement of such metal parts, components or equipment.
In as much as the world is in short supply of water fit for human, animal, vegetal consumption and suitable for industrial processes, cooling, steam generation, washing, dluting, etc., in most instances the salt-containing water available in large supply, is totally useless, unless the salts are removed, in part or almost totally, and useful, low-salt containing water is produced for human, animal, vegetal consumption and for industrial uses. Many methods have been proposed for eliminating, in part, salt from salt-containing water. These methods can be classified into two major groups: methods to remove water from the salt-containing impure water and methods for removing salts from the salt-containing water. The best known first-method processes are reverse osmosis and flash distillation. The best known second-method process is electrodialysis.
The process of reverse osmosis for purification of salt water makes use of plastic films which sorb a solvent but little or none of the solutes contained herein. Reverse osmosis films or membranes strongly absorb water so that electrolytes and solutes cannot permeate the membrane. Accordingly, water can be pumped across the membrane under pressure while solutes are rejected. These membranes are permeable only to water, except that low molcular weight solutes such as methanol and urea can diffuse across them. The membrane hydraulic resistance is high because they do not contain pores as such, but this defect is circumvented by making the membrane very thin and supporting it appropriately. Membranes of this class include cellulose acetate, polyamies and a few other substances. Distillation is a method for water desalination wherein the salt-containing water is heated, evaporated and condensed, thus producing pure water while leaving a high salt-containing residue. This process in as much as it involves evaporation is very energy-demanding and also produces a high salt-containing residue which is corrosive to pipes, pumps, valves, heat exchangers and other metal parts and components.
Another method for water desalination is based on electrodialysis. In the electrodialysis process, salt-containing water streams are passed between alternating pairs of cation- and anion-permeable membranes which control the migration of these ions from one compartment to another under the influence of an electric field. Electrodialysis membranes are particularly useful for the concentration of electrolytes.
Two of the previously described processes, reverse osmosis and electrodialysis make use of membranes. One of the principal advantages to membrane processes is the fact that they do not require a phase change for processes of separation as do flash vaporization and freezing. Accordingly, they have energy requirements not too far removed from those of maximum efficiency or thermodynamic reversibility. They do require very much less energy then do the more conventional separation processes of distillation and freezing. Although equipment costs for electrodialysis are higher than for distillation, its overall costs due to its low energy requirements make the process more acceptable. At the current fuel prices, electrodialysis is strongly favored over flash vaporization. Pressure-driven membranes are not efficient at very high driving pressures (over 1000 psi) because of equipment failure and high cost, so systems where a high osmotic concentration gradients occurs are uneconomical.
Major problems in the development of new energy resources arise during the conversion of coal into convenient fuels or through combustion to produce steam. These processes require substantial amounts of water and most of these fuels are found in arid regions. Water is also required in arid regions to restore strip mining regions to their original state. Purification processes will allow the limited amounts of water available to be reused many times and help make the processes economically feasible.
The survival of millions of people, worldwide, is dependent on the production rate and efficiency in producing food crops. Crop growth is largely dependent on irrigation and multiple re-use of the water often available in limited supply. In most arid parts of the world, including the United States of America, soil fertility is dependent on water supply, therefore the current projects for irrigation if vast arid areas in Arizona, New Mexico, California and other States, where high yield crops are possible if irrigation is conducted. This observation applies also to all inhabited continents where adequate food supply could be produced if adequate water supply was available. Often sea water or underground water is available but unsuitable for irrigation. Therefore economical process to convert these salt-containing water supplies into useable water are in great need.
If the world is to produce energy from the sun via photosynthesis, this would require a vast expansion of irrigation in sun-rich, therefore also often water-deficient areas. Desalination processes will have an important part in removing salts and producing pure water for re-use.
Accordingly, it is clear that, in addition to prior art processes, new efficient and economical processes are needed to provide low-salt contaning water with ever-increasing demand.